Tweaks to classes for events base implementation

Added icon backdrop class, which gives a block element with an icon that grows and shrinks based on viewport (ported from IoT). Added new layout class of centered container which vertically aligns without flex for containers that aren't easily shifted by vertical-align rule. Updated solstice slider to have 4 images at max breakpoint.

Signed-off-by: Martin Lowe martin.lowe@eclipse-foundation.org